deal with qualifiers

[Pascalco]

• Some properties should be used as qualifier, e.g. P1114 (quantity) as qualifier on P912 (has facility). Let the user define a mainsnak and then add the harvested value as qualifier on it.
• Some properties need a mandatory qualifier, e.g. P1082 (population) requires always P585 (point in time) as qualifier. Let the user define a qualifiersnak and add it to the harvested mainsnak